I've got an idea for what to do about the mixing of fill and coal; I'll try it out and post a picture if it works.
I've only fired it up once so far, so the following may not be representative. I found that the clinker tended to build up mostly under the nozzle, but also with some significant lumps above it as well, much the same as the old JABOD. However (and I can't overemphasize how significant this is), it was SUPER EASY to remove the clinker: just build the fire up a bit to melt everything together, kill the blast and allow the clinker to harden up a bit, and fish it out with a poker. Easy peasy. With the JABOD, I had a lot of trouble with the clinker sticking to the adobe (despite the many different sand/clay proportions I tried), and it could take a lot of leverage to get it out (with the attendant risk of something breaking loose and scattering burning coal everywhere). It would almost always bring away pieces of the firebowl. Sometimes I even had to hack the stuff out with a hammer and chisel.
In short, because it's so easy to remove the clinker and the heat of the fire is not going to heat up the clinker or the adobe around the tuyere, it's much easier to keep a good fire going and to have its heat go into the metal where it belongs.
I haven't lost much coal over the edge, but that's just from being careful. It'll be easier once the rim is on. I've decided to add a little gate over the gap in the back rail, which I'll only open up when I need the pass-through for a long piece.
One of the biggest differences between your forge and mine is the depth of the hearth. Mine is significantly deeper, so the fireball sits in front of and slightly above the tuyere, whereas yours (if I remember correctly) is primarily in front. I think if you increased the height of the sides of your forge by about four inches and added a lot more fill, it might work better. I'll try and show you what I mean the next time we get together. (Which might be next week; you around on Tuesday or Wednesday evening?)